High-ductility aluminium alloy part cast under pressure

ABSTRACT

A safety or structural part cast under pressure made of high-ductility aluminum alloy containing, by weight: Si 2-6%; Mg&lt;0.40%; Cu&lt;0.30%; Zn&lt;0.30%; Fe&lt;0.50%; Ti&lt;0.30%, at least one element for reducing adherence to the mold such as Mn (0.3-2%), Cr (0.1-0.3%), Co (0.1-0.3%), V (0.1-0.3%) or Mo (0.1-0.4%), and at least one element for modifying eutectics, such as Sr (50-500 ppm), Na (20-100 ppm) or Ca (30-120 ppm). Other elements may be present in an amount &lt;0.05 each and &lt;0.10 in total, the balance being aluminum. The part exhibits, after T5 tempering at a temperature less than 220° C., a yield strength R p 0.2&gt;110 MPa and an elongation A&gt;10%.

FIELD OF THE INVENTION

The invention relates to the field of aluminium alloys intended for the manufacture of relatively thin aluminium-silicon alloy parts cast by means of die-casting and particularly automobile structural or safety parts.

STATE OF THE RELATED ART

The use of casting aluminium alloys is growing rapidly in the automobile sector, particularly for safety parts such as ground connections and structural components, enabling the vehicle to be lightened. This lightening is all the more important if a high mechanical resistance can be obtained. Known alloys, such as the alloy Al—Si9Cu3Mg make it possible to obtain, in the non-treated temper F, an ultimate tensile strength R_(m) of at least 300 MPa and a yield strength R_(p0.2) of at least 230 MPa. On the other hand, the elongation A does not exceed 2%. However, automobile structural or safety parts require sufficient ductility to absorb energy and prevent rupture in the event of an impact, and also to adapt to various assembly methods.

In addition, the high copper content, while it has a favourable influence on mechanical resistance, renders the alloy susceptible to corrosion. However, a good corrosion resistance, particularly stress corrosion resistance, is required to prevent damage to the part in a corrosive environment such as snow removal salt.

Various alloy formulations have been proposed to meet these requirements. For example, the patent U.S. Pat. No. 3,726,672, filed in 1970 by US Reduction, discloses an alloy for die-cast wheels consisting of (% by weight):

-   -   Si: 11-13.5 Mg: 0.25-0.6 Cu: 0.25-0.6 Mn<0.5 Zn<3 Fe: 0.5-1.5         Cr: 0.25-0.4

The patent U.S. Pat. No. 4,104,089, filed in 1976 by Nippon Light Metal relates to porosity-free die-cast parts for the automobile sector with a high mechanical resistance and shock resistance, consisting of:

-   -   Si: 7-12 Mg: 0.2-0.5 Mn: 0.55-1 Fe: 0.65-1.2

The parts are solution heat-treated between 450 and 530° C., quenched and subjected to artificial ageing for over one hour between 150 and 230° C.

The patent EP 0687742, filed in 1994 by Aluminium Rheinfelden, discloses an alloy for die-casting intended for cast safety parts, consisting of (% by weight):

-   -   Si: 9.5-11.5 Mg: 0.1-0.5 Mn: 0.5-0.8 Fe<0.15 Cu<0.03

The application WO 96/27686, filed in 1995 by Alcoa, discloses die-cast parts for alloy “space-frame” type automobile structures consisting of: Si: 8.5-11 Mg: 0.10-0.35 Mn: 0.4-0.8 Fe<0.50.

The patent AT 404844, filed in 1997 by Aluminium Lend, relates to an alloy for die-casting consisting of:

-   -   Si: 9-12.5 Mg: 0.10-0.60 Mn: 0.30-0.45 Cr: 0.05-0.40 Fe<0.18         Cu<0.05 Zn<0.1 Ti: 0.01-0.20

The applications EP 0992601 and EP 0997550, filed in 1998 by Alusuisse, disclose the die-casting manufacture of alloy parts consisting of:

-   -   Si: 9.5-11.5 Mg: 0.1-0.4 Mn: 0.3-0.6 Fe: 0.15-0.35 Ti<0.1 Sr:         90-180 ppm if applicable Cr: 0.1-0.3 Ni: 0.1-0.3 Co: 0.1-0.3

The parts are subjected to a partial solution heat treatment between 400 and 490° C.

These various alloys require, to achieve the required mechanical resistance, a heat treatment with a solution heat treatment and quenching, which induces significant deformations of the parts which need to undergo rectification, resulting in a significant increase in the cost price.

The patient U.S. Pat. No. 6,132,531, filed in 1997 by Alcoa, relates to an alloy for die-casting, particularly intended for “space-frame” type automobile bodywork structural nodes consisting of: Si<0.20 Fe<0.20 Mg: 2.80-3.60 Mn: 1.10-1.40 Ti<0.15 Be: 0.0005-0.0015. Good mechanical properties are obtained with no heat treatment of the cast parts. The lack of silicon affects the castability of the alloy.

PURPOSE OF THE INVENTION

The purpose of the invention is to provide aluminium alloys for the die-casting of automobile structural and safety parts offering a sufficient mechanical resistance without necessarily requiring a complete T6 or T7 type heat treatment, a high ductility, a good corrosion resistance, a good castability and enabling the manufacture of mass-produced parts under economically acceptable conditions.

SUBJECT OF THE INVENTION

The invention relates to a die-cast safety or structural parts made of ductile aluminium alloy consisting of (% by weight):

-   -   Si: 2-6 Mg<0.40 Cu<0.30 Zn<0.30 Fe<0.50 Ti<0.30,     -   at least one element intended to reduce the adherence on the         mould such as Mn (0.3-2%), Cr (0.1-0.3), Co (0.1-0.3), V         (0.1-0.3) and Mo (0.1-0.4),     -   and at least one eutectic modifying element, such as Sr (50-500         ppm), Na (20-100 ppm) and Ca (30-120 ppm),     -   other elements <0.05 each and <0.10 in total, the remainder         being aluminium,     -   showing after artificial ageing T5 at a temperature below 220°         C., a yield strength R_(p0.2)>100 MPa and an elongation A>10%.

Preferentially, the following limit values are targeted for the composition:

-   -   Si: 3.5-5 Mg: 0.05-0.20 Mn: 0.7-1.5 Ti: 0.05-0.15 Cu<0.10         Zn<0.10.

DESCRIPTION OF FIGURES

FIGS. 1 a, 1 b and 1 c represent the respective variation of the ultimate tensile strength, yield strength and elongation as a function of the silicon contents, for different magnesium contents, on 2.5 mm thick sample plates die-cast with vacuum assistance and no heat treatment (F temper).

FIG. 2 shows the mechanical characteristics of die-cast parts SP and in gravitational casting dies Coq for 7% silicon alloys as a function of their magnesium content.

DESCRIPTION OF THE INVENTION

The invention is particularly based on the observation that, by lowering the silicon content with respect to the alloys of the prior art intended for the same applications, it is possible to obtain for die-cast parts an interesting compromise between mechanical strength, particularly in the single aged temper T5 and ductility, while retaining an acceptable castability, and an absence of hot cracks and contraction cavities. The silicon content is at least 2% to retain a good castability, and not more than 6% to obtain a high ductility in the T5 yemper. It is preferentially between 3.5 and 5%. The influence of the silicon content on the yield strength and the elongation is illustrated by FIGS. 1 a and 1 b in the F temper (non-treated) and FIG. 2 in the T5 temper, which particularly show the rapid reduction in elongation when the silicon content increases.

Magnesium has the same effects as silicon by forming Mg₂Si particles with it, which have an hardening effect during the heat treatment. However, the applicants surprisingly observed that at cooling rates specific to the die-casting of thin parts, this hardening partially remains in the absence of solution heat treatment and quenching, probably due to a supersaturation effect of the solid aluminium solution. It is thus possible to limit the magnesium content to 0.4, or even to 0.3 or 0.25%, which improves the elongation, while retaining a good yield strength.

Iron, manganese, chromium, cobalt, vanadium, molybdenum and nickel form, individually or in combination with aluminium, embrittling intermetallic compounds, and the content thereof must be limited. However, in the event of a high die release cooling rate, the embrittlement is lower, since these compounds have a smaller size and a more favourable shape, Conversely, the same elements help reduce the “adherence” in the die, by reducing the chemical potential of the alloy with respect to the steel. As iron has an unfavourable influence on elongation and must be limited to 0.5%, and preferentially 0.2%, it is essential that, in addition to iron, at least one of the other elements is present. If this element is manganese, its content must be between 0.3 and 2%, and preferentially between 0.7 and 1.5%. Titanium, combined with boron, is a refining agent of the solid aluminium solution by reducing the grain size of the primary particles. In addition, it increases the volumetric contraction supply capacity during solidification, which helps improve the compactness of the parts.

Copper must be kept below 0.3%, and preferentially 0.1%, to prevent susceptibility to corrosion, and because it reduces elongation. Lead, tin and antimony inhibit the action of the modifying elements.

Eutectic modifying or refining elements, such as strontium, sodium and calcium, modify the size and shape of silicon inserts by giving them a fibred structure. They may also act as refining agents for some intermetallic compounds.

Die-cast parts, with or without vacuum assistance, using the alloys according to the invention, may be used without a heat treatment (F temper), or, on the other hand, be subjected to a complete heat treatment T6 or T7 comprising a solution heat treatment, quenching and artificial ageing, or a treatment T3 with natural ageing after quenching. They are particularly well-suited to a single artificial ageing treatment (T5 temper), at a temperature below 220° C., lasting between 15 min and 1 hour, it being possible to carry out said treatment, for automobile parts, during the paintwork baking operation, which is generally carried out at a temperature between 150 and 220° C.

The alloys according to the invention can be used to produce parts with a good mechanical resistance and a high elongation, resulting in a good shock resistance and rendering assemblies requiring a high ductility, for example crimping, possible. They also show a good TIG, MIG or laser weldability and a good compatibility with aluminium 6000 alloys used for bodywork. The alloys have a low oxidisability in the liquid state, a good reyclability and a low melting loss during waste remelting.

EXAMPLES Example 1 Effect of the Composition on Static Mechanical Characteristics

Sample plates were produced by die-casting, with vacuum assistance (residual die pressure of 80 hPa) made of 9 different alloys A to I, wherein the composition is given in table 1. The format of the plates was 120×120 mm and they are 2.5 mm thick. Casting was carried out on a press with a locking force of 3200 kN, with a piston injection rate of 0.7 m/s. The temperature of the metal in the furnace was 780° C. TABLE 1 Alloy Si (%) Fe (%) Mn (%) Mg (%) Ti (%) Sr (ppm) A 2.85 0.12 0.67 0.11 0.15 220 B 2.90 0.12 0.68 0.16 0.15 205 C 2.85 0.13 0.67 0.21 0.15 185 D 4.90 0.13 0.68 0.09 0.16 290 E 4.90 0.13 0.68 0.14 0.16 250 F 4.90 0.13 0.68 0.21 0.16 210 G 6.90 0.14 0.66 0.09 0.17 236 H 6.95 0.14 0.66 0.14 0.17 195 I 6.95 0.14 0.67 0.20 0.17 192

In these non-heat-treated plates, tensile test pieces were machined and the ultimate tensile strength R_(m) (in MPa), the yield strength at 0.2% elongation Rp0.2 (in MPa) and the elongation A (in %). The results (means of 10 test pieces) are given in table 2 and in FIGS. 1 a, 1 b and 1 c. TABLE 2 Alloy R_(m) R_(p0.2) A A 206 88 20.4 B 209 93 20.7 C 212 96 19.5 D 232 98 17.1 E 239 106 15.0 F 247 113 15.8 G 255 107 12.8 H 267 121 11.5 I 278 127 12.0

It is observed that, for alloys between 3 and 7% silicon and between 0.1 and 0.2% magnesium, the ultimate tensile strength and the yield strength increase with the silicon content, and the elongation decreases. The yield strength also increases with the magnesium content, while the effect of the magnesium on elongation is not significant.

Example 2 Effect of Cooling Rate

Test pieces machined to a diameter of 13.8 mm in accordance with the standard NF A 57-102 were cast in gravitational casting dies in alloys containing 7% silicon and 0.13 and 0.20% magnesium, respectively and the mechanical characteristics were compared to those of example 1 for alloys G, H and I. The results are given in table 3 and in FIG. 2. TABLE 3 Alloy Casting R_(m) R_(p0.2) A G Die-cast 255 107 12.8 H Die-cast 287 121 11.5 I Die-cast 278 127 12.0 G Gravitational 164 57 22.6 H Gravitational 181 74 22.0 I Gravitational 195 85 16.4

It is observed, for gravitational die cast parts, with a much lower cooling rate, that the ultimate tensile strength and the yield strength are much lower, particularly in the event of low magnesium contents, and the elongation much higher. 

1. Die-cast safety or structural part made of ductile aluminum alloy consisting essentially of, by weight: Si: 2-6%; Mg<0.40%; Cu<0.30%; Zn<0.30%; Fe<0.50%; Ti<0.30%; at least one element for reducing adherence to a mold selected from the group consisting of Mn 0.3, 2%, Cr 0.1, 0.3%, Co 0.1-0.3%, V 0.1-0.3% and Mo 0.1-0.4%, and at least one eutectic modifying element, selected from the group consisting of Sr 50-500 ppm, Na 20-100 ppm and Ca 30-120 ppm, other elements <0.05% each and <0.10% in total, the remainder being aluminium aluminum, said part having after artificial ageing T5 at a temperature below 220° C., a yield strength R_(p)0.2>100 MPa and an elongation A>10%.
 2. Part according to claim 1, characterised in that the silicon content is between 3.5 and 5%.
 3. Part according to claim 1, characterised in that the magnesium content is between 0.05 and 0.25%.
 4. Part according to claim 1, characterised in that the titanium content is between 0.05 and 0.15%.
 5. Part according to claim 1, characterised in that the copper content is less than 0.10%.
 6. Part according to claim 1, characterised in that the iron content is less than 0.20%.
 7. Part according to claim 1, characterised in that the zinc content is less than 0.10%.
 8. Part according to claim 1, characterised in that the manganese content is between 0.7 and 1.5%. 